The detection of vapor species present in the atmosphere is of great interest in numerous applications. It is the basis of olfaction in the biological world. Man's sense of smell is relatively weak, so the most demanding aspects of this function must be taken up by various artificial detectors. Examples of the actual or potential usefulness of vapor detectors are clear in various fields, such as food and aromas; medical diagnosis; security applications (explosive detection, person recognition, identification of pathogens, etc.); monitoring of atmospheric pollutants, etc.
The detection of vapors becomes increasingly difficult when they are present at decreasing concentrations in a gas (i.e. the atmosphere) for two reasons. First, one needs an increasingly more sensitive detector. Second, the number of competing vapors present in the atmosphere (or other complex gaseous mixtures) at concentrations comparable to those of the target vapor increases rapidly as the concentration of the target vapor decreases. Consequently, the discriminating power or resolution of the detector must increase along with its sensitivity. Few detectors have been available capable of sensing vapors at concentrations below parts per billion (1 ppb=10−9 atmospheres of partial pressure). This is a concentration range at which animals such as dogs are believed to often be still sensitive, and at which many useful vapor detection tasks can be performed. Many other applications require detecting still smaller concentrations down to parts per trillion (1 ppt=10−12 atmospheres of partial pressure) or parts per quadrillion (1 ppq=10−15 atmospheres of partial pressure). This is necessary, for instance, to detect low vapor pressure substances such as plastic explosives. The most advanced state-of-the-art artificial detectors of substances in the atmosphere are used for security screening of explosives in the civil aviation field, and are called Explosive Trace Detectors, or ETDs. ETDs are based on ion mobility spectrometers (IMS), and though they claim sometimes the ability to detect low volatility materials, generally do not do so directly in the gas phase through olfaction. Rather, detection is achieved by swiping a target surface with a cloth in the hope of collecting some condensed sample of the target substance (typically one or several microscopic particles). The cloth is then introduced into a heated region where any particle present is volatilized, ionized and detected.
The most advanced state-of-the-art detectors for low volatility substances are the explosive detectors used for airport explosive screening, since they target, among others, plastic explosives, whose vapor pressure is extremely low. All airport ETD detectors currently deployed rely on high speed gas jets or on swabbing for sampling from surfaces, with the obvious goal of dislodging small particles from the person or object probed. A device relying on vapor analysis would simply sample the gas from the vicinity of the suspected point, without the help of auxiliary jets which necessarily greatly dilute the sample vapor.
The particle collection occurrence is highly randomized in real environment. The amount of target substances (or contaminants) that present detectors measure depends on the size and number of particles collected, which is highly variable between tests. As a result, the signals produced by particle based detectors are highly scattered, and the operator needs to increase the detection thresholds accordingly. As a result, the Probability of Detection (PoD) and the False Alarm Ratio (FAR) of these detectors become very poor.
As reported in June 2010 by Mr. Steve Lord, Director Homeland Security and Justice Issues, before the House of Representatives [Subcommittee on Transportation Security and Infrastructure Protection, Committee on Homeland Security, GAO-10-880T; see web page http://www.gao.gov/new.items/d10880t.pdf)], there is currently no technology approved or qualified by TSA to screen cargo once it is loaded onto a ULD pallet or container. Pallets and containers are common means of transporting air cargo on wide-body passenger aircraft. Prior to May 1, 2010, canine screening was the only screening method, other than physical search, approved by TSA to screen such cargo. However, TSA officials still have some concerns about the effectiveness of the canine teams, and effective May 1, 2010, the agency no longer allows canine teams to be used for primary screening of ULD pallets and containers. The Transport Security Administration (TSA), (see web page http://www.tsa.gov/assets/pdf/non_ssi_acstl.pdf) has produced a document entitled “TSA Air Cargo Screening Technology List (ACSTL)”, which defines the technologies accepted for air cargo screening. In the first page of this document, the TSA declares the following: “Despite what some manufacturers advertise, TSA has not approved any equipment for any ULD screening. The maximum size cargo configuration that may be screened is a 48″×48″×65″ skid.” A ULD is a well known acronym in the air cargo community; it stands for “Unit Load Device”, and is a pallet or container used to load luggage, freight, and mail on wide-body aircraft. It can be understood as a precise definition of specific containers used for aircrafts. One can therefore conclude that no detection method presently exists for explosive screening of containers for air cargo, neither canine, nor technology-based.
We have for some time expressed our view that vapors can be detected at ppt concentrations, but have met wide skepticism from the experts. No instrument comparable in sensitivity with the one described in this invention has been available to others to discover what it takes to go from the barely credible ppb detection level down to the ppt level, not to speak of the ppq sensitivities aimed at in this invention.
In contrast, the focus of the present invention is on the detection of species present in the gas phase as vapors, rather than of species present on a surface in solid or liquid form. Vapors diffuse smoothly and produce much more repeatable signals that allow the operator to reduce detection thresholds. This pure gas phase approach had for a long time been considered as hopeless for detection of low volatility substances. However, we have recently demonstrated its suitability in certain situations to detect volatiles at concentrations below 1 ppt (Mesonero et al. 2009). Initially we used a sensitive mass spectrometer having an atmospheric pressure source (API-MS) preceded by an ionizer. In a preferred embodiment, vapor ionization was achieved by putting the gas to be analyzed in contact with a cloud of charged particles produced by electrospray (ES) ionization (Fenn et al. 1989). Although a sensitivity to sub ppt concentrations was unprecedented, we found that detection at these concentrations still made use of a very large number of individual molecules, on the order of 108. A lowest detection level in the ppt range therefore implies a sensing efficiency of 10−8. The room and the need for substantial improvements are therefore clear. We reported two kinds of barriers to achieving improved sensing efficiencies. One was a low vapor ionization probability of the order of 10−4 (i.e., only one in 104 vapor molecules contained in the sample volume passed through the ionizer and become an ion sucked into the mass spectrometer). This difficulty has been alleviated by an improved Secondary Electrospray Ionization (SESI) scheme, taught in U.S. patent application Ser. No. 12/686,669 by Vidal. The other obstacle was the impossibility to distinguish the signal of a target molecule from that of other species having a similar chemical signature (chemical noise). The use of Differential Mobility Analyzer (DMA) in tandem with MS-MS also alleviated this problem. As described in U.S. Pat. No. 7,855,360, the use of these three narrow band ion filters in series (DMA-MS-MS) produces a drastic reduction in chemical noise, without significant penalty (with respect to an analysis based on a single or a triple quadrupole MS alone) in terms of analysis time and ion transmission.
The first experimental system incorporating a Sampler pre-concentrator, a desorber, and a SESI-DMA-MS-MS ionizer and analyzer used for our preliminary explorations, and which is incorporated in the present invention, was unique in the world in its ability to sense vapors at concentrations below 1 ppt. However, systematic work with this apparatus and its subsequent improvements have provided for the first time a clear picture of a number of previously unsuspected difficulties:
i) Sampler contamination. Consider for concreteness an attempt to detect the presence of the plastic explosive RDX as a vapor in the atmosphere. Its equilibrium vapor pressure at ambient temperature is quoted to be in the range of a few ppt (This value, however, must be taken as provisional, as no reliable method to measure such a low vapor pressure has been available prior to the development of our detector). There are many thousands of other vapors present in the ambient at higher partial pressures that could produce background signals. However, we were able to determine confidently after DMA-MS-MS analysis that at least part of the background signal remaining in the detector after removal of the source of RDX vapor was due to contamination of the analyzer with RDX itself. This forced us to review with exquisite care all possible sources of RDX contamination resulting from previous tests with RDX samples. We had of course followed conventional practices to avoid contamination from samples. In particular, all gas lines were heated to remove any RDX deposited and absorbed on the walls of the system during a measurement with RDX vapor. However, at the unusual sensitivity levels of our instrument, these common practices were clearly insufficient. Eventually we discovered several sources of contamination in the sampling system. Most important was the fact that, upon removing the filter from the sampler, some RDX collected on the filter was transmitted to the surface of the filter holder and remained there as a contaminant. Then, on introducing a new clean filter to collect a new sample of ambient air, part of the RDX contaminant left on the sampler was transferred to the new filter. Sometimes vapors or particles of RDX were collected on the walls of the sampler. In this case there was no direct or indirect contamination of the filter via condensed phase contact. But the finite vapor pressure of the adsorbed RDX molecules and particles led still to a sufficient release of RDX vapors from the contaminated sampler surfaces into the sampled gas and from this gas into the collecting filter. These forms of contamination were always indirect, and therefore weak and hard to detect. But they were certainly sufficient to preclude detection levels in the ppq range, and could be unambiguously identified with our sensitive instrument, particularly after careful cleaning and decontamination. Another subtler form of indirect contamination was subsequently discovered. Upon completing a sampling experiment on a particular filter, the pump sampling the air at the exit of the sampling system was stopped. Occasionally, however, ambient gas entered backwards into the system, moving upstream from the pump into the sampler. This backward flow resulted in contamination of the sampler very much as when RDX contamination came from upstream.
ii) Ambient contamination. Many other conventional practices need to be refined for vapor detection at ppq levels. Military installation rooms presumed to be clean, where explosives had been manipulated in the past, were found to be heavily polluted. We therefore have often installed the analyzer in its own external cabin, whose cleanness we could more easily control. Even so, we found that this cabin tended to become contaminated after several days of analysis of samples. As a consequence, background levels measured rise and so does the detection threshold. Similar problems may be encountered in various operational scenarios. For instance, when sampling large volumes from a truck or other large containers, the effluents from the sampling pump, if improperly handled, may artificially contaminate a neighboring truck or cargo container.
An important aspect in the development of ppq level detectors is their test with standards. This invention is therefore concerned not only with the improvement of unusually sensitive vapor detectors, but also with the development of methods to rigorously test such detectors. Careful avoidance of contamination is as relevant in such testing grounds as in the analyzer cabin just discussed. Contamination can arise from the most unsuspected sources. For instance, we find that dogs trained to detect RDX and believed to be capable of actually detecting it, are in fact detecting trace quantities of other far more volatile explosives such as TNT or EGDN, accidentally present in the training sample of RDX. Careful examination of the sample handling procedure confirms the likelihood of slight cross contamination between the low volatility and the high volatility test samples. The sample may be polluted even without physical contact, for instance when a volatile and an involatile sample are placed in a common container, since the volatile sample can diffuse through the gas and adsorb on the surfaces of the container or the other solid samples held within it. One can hence easily envision situations where certain seemingly effective detectors could in fact have been for long periods entirely ineffective with uncontaminated low volatility samples. Standards for such tests need therefore to be handled with unusual care. For biological or other non-chemically specific (i.e. non-MS) detectors, testing samples must be analyzed periodically with chemically specific detectors in order to confirm the lack of such cross contamination. Many other subtle contamination schemes have been found during our studies. For instance, testing for explosives is frequently carried out in specialized facilities, often in closed buildings with unusually large volumes and little ventilation. Boxes of various shapes and volumes containing substantial quantities of explosive sample are then introduced into such buildings, and the interior volumes of such boxes are sampled in various forms. If the gas leaving the pump of the sampler pre-concentrator exits directly into the testing building, it contributes to its background, since the collector collects typically less than half of the vapor sample.
iii) Particulate associated chemical noise. Most samplers used for trace detection are primarily based on collecting and detecting particles. The reason is that their modest sensitivity precludes direct vapor detection, whence, their only hope to detect low vapor pressure substances is to collect them in the form of particles, typically carrying far more mass than the vapor molecules in the gas. Conventional samplers often use a pre-filter. Since particles are the main source of detected low vapor substances, the aim of this filter is evidently not to remove small particles potentially carrying the target substance. Its purpose is rather to remove relatively large organic objects, such as insects or their fragments, whose capture in the filter would overwhelm and often seriously contaminate the analytical system. The size of the particles typically intercepted by the pre-filter in such samplers varies from one to the other. Generally, explosive-bearing particles are believed to have sizes from a few microns to hundreds of microns. Explosive vapors are generally sticky, so they tend to attach to particulate matter, and by aerodynamically removing such particles from a surface one hopes to collect a measurable amount of explosive. It is therefore desirable to collect as much as possible of the suspended particles to maximize the amount of explosive collected on their surface. This desire must of course be moderated somehow to minimize the risk of capturing large chunks of organic matter, which upon heating would release inordinate doses of volatile contaminating materials. A reasonable cutoff size for the pre-filter is therefore certainly to remove debris in the mm size range. More conservatively, samplers tend to remove even smaller particles, typically down to 100 μm. One rational for this lower size range is that larger particles settle rapidly into the floor, are harder to bring into the gas phase, and after being aerosolized tend to settle along the sampling lines or impact on their curved sections. The potential advantage of sampling particles larger than 100 μm therefore rarely compensates for the associated contamination risks. Consequently, a typical pre-filter will retain particles larger than 100 μm, and pass most of the smaller particles for their potential content of the target substances to be captured on the main filter. In other words, sampling systems for low vapor pressure substances typically rely on particle collection in the size range below 50-100 μm. In many detection applications, one must set a threshold concentration to launch an alarm. In order to avoid false alarms, this threshold must be higher than the background. But if the background rises (even rarely) every time a big particle of chemical interfering species is captured, the threshold needs to be raised above this value. And under such circumstances the lower detection level of the system is not set by the real sensitivity of the detector, but by the high threshold required to reduce the FAR.
iv) Electrospray limited temperature. Following detection of a low vapor pressure substance, the detector often tends to give a positive leftover signal for that substance. This leftover or memory effect decreases steadily over time, but does not fall to zero for extended periods due to adsorption into and slow release of the vapor from the walls of the analyzer. In order to accelerate the complete decay to zero of this leftover signal, IMS systems and other analytical devices often operate at elevated temperatures, at which adsorbed vapors are rapidly released into the gas. It is accordingly important to heat all system components where such adsorption could take place.
One of the most effective vapor ionization sources available, often referred to as a Secondary Electro-Spray Ionizer (SESI), simply mixes the sample vapors with a cloud of charged droplets of a relatively clean volatile solvent (Fernandez de la Mora 2011). Other more common ionizers rely on an electrical discharge or a radioactive source, often based on a foil of 63Ni. These two later ionizers are readily heated, but not the first. Indeed, an electrospray source requires that the electrosprayed solvent be in liquid form, which forces an operational temperature below its boiling point. It is important that all parts of the ionizer in contact with the sample gas be hot, ideally at a temperature in the range of 150° C., preferably higher. This is well above the boiling point of most common solvents, certainly those previously used for SESI applications. Wittmer et al. (1994) have developed a water cooled electrospray source, which has been used by Wu et al. (2000) for vapor ionization. Still, the sample gas cannot flow over the water-cooled regions (including the tip of the Taylor cone where the ionizing spray drops are produced), as cold surfaces would trap the gas and lead to the undesirable memory effect one seeks to avoid via heating. Mixing between the sample gas and these charged drops (or the ions they release after drop evaporation), must therefore occur downstream from the ion source, in an uncooled region. However, as shown by Fernandez de la Mora (2011), ionization is considerably more effective when the vapor to be ionized is present at the very tip of the Taylor cone. Achieving this more efficient condition hence requires an approach free from cooled surfaces.
v) Contamination from the electrospray solvent and nitrate-based explosives. SESI-based ionization of vapors offers certain advantages over corona ionization. First, because of the absence in SESI of energetic and potentially reactive phenomena taking place in the corona region, where impurity species may be created from preexisting vapor species in the sample. Another advantage of SESI is that certain additives introduced in the electrosprayed liquid readily produce desired reagent ions in the gas phase. For instance, a good option for ionizing many explosives is to produce halogen ions (i.e. Cl−) by seeding them into the electrospray solvent in salt or acid form (i.e., NaCl or HCl). The acid form is generally preferable because acids are far more volatile than salts so the spray of drops produces fewer solid residues. However, we find that commercial aqueous HCl is typically supplied with considerable proportions of impurities of sulfates, nitrates and other inorganic anions. This is true even for specialty reagents used for metal analysis, which have indeed very small levels of metal cation contaminants, but still contain substantial quantities of anions such as nitrate. The NO3− ion is therefore abundantly formed upon electrospraying such solutions. This impurity ion interferes with the detection of nitrate explosives, typically represented in the gas phase by its volatile decomposition products NO3H and NH3. Indeed, NO3H vapor ionizes into NO3− in negative ionization, so solution and gas phase nitrate are indistinguishable, posing a difficult obstacle to the successful sniffing of ammonium nitrate. A first step to the solution of this serious interference is to identify nitrate-free halides, which can indeed be obtained. This is however insufficient to safely sniff ammonium nitrate explosives because other abundant sources of nitric acid exist in the atmosphere. It is in fact well known that ammonium nitrate itself forms naturally in the atmosphere over polluted sites where ammonia and nitric acid preexist at sufficient concentration.
The problem of detecting vapors in the ppq level is currently not solved. It is commonly believed by those skilled in the art that detection of vapors at the ppq level in real environments is impossible. Moreover, substances with low vapor pressures such as plastic explosives are believed to be non-volatile, and their detection is based on particle collection and detection, however, particle detection occurrence is highly randomized, and particle based explosives detection is not functional in the real field.
Consequently, one purpose of this invention is to teach how to detect atmospheric vapors by means of a system built from a Sampling system which acts as pre-concentrator, a primary filter which retains vapors and vehicles information from the sampling system to the Analyzer, and an Analyzer incorporating a thermal desorber, a SESI ionizer, and a DMA-MS-MS analyzer. Other purposes of the present invention are to teach:                (i) how to prevent sampler contamination,        (ii) how to prevent contamination of the analyzer through the atmospheric background during operation,        (iii) how to prevent chemical noise produced by aerosol particles present in the air to be analyzed,        (iv) how to prevent contamination produced by the electrospray used in some embodiments of the invention, and        (v) how to operate a SESI at high temperature,        
so as to detect low volatility species directly in the gas phase and at concentrations down to the ppq level.